Field of the Invention
The invention relates to an LED device including at least one LED chip having a front contact metallization disposed on a light exit surface of a light-emitting member, and a rear contact metallization disposed on a side of the light-emitting member opposite the light exit surface.
Such LED devices are known, for example, from an article entitled "Das LED-Display, ein vielseitiges Anzeigenelement" [The LED Display, a Versatile Display Element], by H. Cuno and A. Heider, in elektronik industrie 1-1991. Described therein is an LED segment in which an LED chip having a front contact metallization and a rear contact metallization is fastened with its rear to a first electric connecting part. The front contact metallization is provided with a contacting wire which connects the front contact metallization to a second electric connecting part.
LED devices of the type mentioned at the outset are also known in which a plurality of LED chips are fastened with their rear contact metallizations in accordance with a prescribed array to conductor tracks of a printed circuit board that interconnect the rear contact metallizations in an electrically conducting manner in rows or columns. The front contact metallizations are constructed as bonding pads and are interconnected in an electrically conducting manner in columns or rows through the use of bonding wires. A transparent plastic plate, which is built into a plastic housing together with the printed circuit board, is usually disposed over the LED chips in order to protect against mechanical damage.
In the case of so-called intelligent LED display devices which are commercially available, for example, under the designation "Intelligent Display" and are presently constructed in accordance with the principle described above, the rows and columns are connected to an electronic driving circuit which supplies the LED chips with current or current pulses (pulse code modulation) in such a way that they light up in accordance with a prescribed pattern. The driving circuit can, for example, be attached to the rear of the printed circuit board, or externally as well. Such intelligent LED display devices are used, for example, in mobile telephones for displaying telephone numbers and operating states, since they are self-luminous and therefore do not require any additional illumination such as is necessary, for example, in the case of LCD displays.
The known LED devices described above are produced through the use of conventional die bonding and wire bonding techniques. The front contact metallizations are constructed as bonding pads and thereby cover a part of the light exit surfaces of the LED chips. In order to ensure that the ratio of the light exit surface to the bonding pad surface, and thus the light yield from an LED chip does not become too small, it is not permitted with the conventional bonding techniques presently available for the cross section of the LED chip to fall below 0.2.times.0.2 mm.sup.2 in plan view.
The costs, the weight and the dimensions of the above-mentioned LED devices depend substantially on the size of the LED chips. Likewise, the luminous spot density (number of light-emitting diodes per unit area), and thus also the image quality of the above-described LED displays depend substantially on the size of the individual LED chips and their mutual spacing. It is therefore particularly worth the attempt at reducing the overall size of the LED chips.
A reduction in the chip surface, for example to 0.15.times.0.15 mm.sup.2 would require a conspicuous reduction in the bonding pad surface, in order to prevent the light yield from the LED chip from being drastically worsened at the same time. However, that would entail substantial technical problems in the case of the use of conventional bonding techniques.